The present invention relates to systems for transporting and delivering ice particles at high velocity onto a workpiece for cleaning or other treatment of the workpiece.
It is commonly known to blast a workpiece with a particulate abrasive that either melts or sublimes at room temperature for cleanly dissipating the abrasive subsequent to its use, thereby avoiding contamination of the workpiece or its environment. The abrasive can be frozen water, typically called "ice", solid carbon dioxide, typically called "dry ice", or combinations comprising one or both of these materials. One well known process for forming the particulate as dry ice is disclosed in U.S. Pat. No. 4,389,820 to Fong et al, wherein liquid CO.sub.2 is dispensed and frozen in a snow chamber, the snow falling into a planetary extruder die mechanism where it is compacted into pellets by being forced through radial holes of a ring-shaped die, the length of the pellets being defined by structure that fractures the material by partially blocking the exit paths from the die. The pellets can be dispensed directly upon formation or they can be stored and/or transported for use upon demand in a hopper or the like. Among the problems in this art are the following:
1. The size of the particles greatly affects blasting quality and efficiency, large particles being desirable for breaking through crusty contamination of the workpiece, smaller particles being needed for reaching small features of the workpiece, and different mixes of sizes are needed for many jobs;
2. It is more difficult to make small pellets than big ones;
3. It is difficult and expensive to adjust the particle size by changing the diameter of the pellets, in that the die is difficult to replace and the multiplicity of radial holes are expensive to produce;
4. Although some adjustment in particle size is possible by changing the length between fractures of the emerging material, the length must be maintained at near twice the diameter for uniform particle size-shortening the distance between the fractures produces greater relative variation in the length of the pellets, and attempts to make the length very short seriously degrades the integrity of the particles while subjecting the die to clogging;
5. The particles are subject to degradation by subliming, by melting, and by abrasion or pulverization during transport to the workpiece, these mechanisms having increasingly adverse effects as the particle size is reduced; and
6. The delivery of particles at very low temperatures rapidly cools the workpiece, often with undesirable effects. For example, when the workpiece is cooled below the dew-point, moisture collects thereon subsequent to the treatment, the moisture tending to attract other contaminants and thereby defeating the purpose of the treatment.
Thus there is a need for a delivery system for hygroscopic or deliquescent particulate that delivers the material at high velocity and low temperature with precise control of particle size distribution. There is a further need for such a blasting system that avoids excessive cooling of the workpiece without degrading the particulate.